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Making green roofs work in Australia

When Claire Farrell couldn’t find quite the right plants to grow on rooftop gardens in Australia’s hot, dry climate, she went bush.

Farrell, a modern-day plant-hunter, set off for a remote Victorian National Park to investigate which plants were thriving on rocky outcrops with shallow soils (conditions similar to green roofs). She identified a range of species then ran an experiment at the University of Melbourne’s Burnley campus.

The results showed four plants were well-suited. These species have now been planted across roofs in Melbourne. Farrell thinks they could be part of solving the puzzle of growing green roofs in Australia.

‘Green roofs’ are gardens planted out on rooftops. They’re popular in Europe, particularly Germany, and in US cities like Portland and Chicago. Popular inhabitants are succulents, flowers, vegetables and native species.

Green roofs haven’t taken off in Australia, says Farrell, a Lecturer in Green Infrastructure in the University of Melbourne’s Science faculty. “They’re not as widespread as overseas,” she says. “It’s hotter and drier here … you can’t just translate what’s been done in temperate parts of the US and Europe to Australian conditions and expect it to work. You’d have a lot of dead plants.”

So researchers at the University’s Burnley campus, located on a bend of the Yarra River in suburban Melbourne, built Australia’s first research green roof. It opened in 2012. People can walk from the staff room out onto the second-floor roof, which contains more than 200 species, paths, seats and an umbrella.

“We get a lot of butterflies and bees and birds, just because there’s so many different plants out here,” says Farrell. “The general public love it. People will say ‘can’t we just put that everywhere?’”

Farrell’s plant-hunting expedition to Terrick Terrick National Park, near the Murray River, came about because a key reason to plant a green roof is to reduce stormwater runoff. Heavy rain rushes straight off hard surfaces into waterways, causing erosion and spreading pollution. Gardens soak up some of the rain, reducing runoff.

The catch is that the best survivors on green roofs are succulents (which store water in thick, fleshy leaves or stems) – but they absorb little water when it rains. Farrell wanted to find native plants which can survive drought but soak up plenty of water when it’s wet. Her four best species – three monocots (grasslike plants) and a herb – do just that.

“That work is pretty groundbreaking, prior to that it was all about survival,” she says. “No one really thought about balancing water use and survival.”

There’s plenty of other research happening on Burnley’s demonstration roof. It includes sections with shallow soil and deeper soil, irrigated and non-irrigated sections, native and exotic species. There’s even tomatoes and thyme, along with Farrell’s thriving monocots. Students taking the Specialist Certificate in Green Roofs and Walls study there.

“It’s hotter and drier here … you can’t just translate what’s been done in temperate parts of the US and Europe to Australian conditions and expect it to work. You’d have a lot of dead plants"

Burnley researchers found the cooling effect of a rooftop garden (soil and plants reflect heat, and provide shade and insulation) can cut air-conditioning use by 38 per cent. A PhD student found people preferred green grassy vegetation on a green roof, and that working nearby improved concentration.

The Burnley team has gone on to design green roofs for five commercial and University buildings, including the first native grassland roof in the southern hemisphere.

But before householders race out to plant a rooftop garden, beware. Farrell points out most existing houses aren’t strong enough to carry the weight; new houses work better. With summer temperatures regularly in the 40s, the plants have to be right, as does the soil (Farrell has been experimenting with different soil additives; a form of charcoal stores water well and helps plants survive).

But if a green roof seems too hard, the Burnley team also does green walls and facades.